Polyester films are widely used in the fields of magnetic recording medium, electrical insulation material, packaging material, etc., owing to the excellent thermal and mechanical properties.
The development of a magnetic recording medium of digital recording system is recently in progress to enable the recording and reproduction of picture and sound in improved quality compared with conventional analog recording system. The recent progress of the medium for the digital recording system aims at high capacity, high recording density, long recording time and compact-sized cassette as well as high picture quality and sound quality and the requirement on the properties of the base film is increasing according to the improvement of the above qualities.
It is necessary to decease the thickness of a magnetic tape and, accordingly, the thickness of a base film for prolonging the recording time and reducing the cassette size. However, the rigidity of a tape decreases with decreasing thickness of a magnetic tape to deteriorate the contact of the tape with a magnetic head in recording and reproduction, which makes difficult to get high picture quality and sound quality.
The track pitch is selected to be extremely narrow especially in the data-storage use adopting a linear track system such as QIC (Quarter Inch Cartridge) and DLT (Digital Liner Tape) for getting high capacity and high density. The narrow track pitch produces a problem of error by the shift of track caused by the dimensional change in the transversal direction of the tape.
These dimensional changes are supposed to be caused by the changes in the temperature and humidity in the use of the magnetic tape and the variation of tape width induced by the variation of the transport tension of the tape inherent to the recent compact-sized drive. The extent of the influence of each factor is not yet clear to produce a problem in the case of increasing the capacity and density of a magnetic tape.